YOLK-SAC OF EARLY EMBRYO. 



IOI 



only, representing about one-tenth the bulk of the unsegmented egg, is reserved 

 for the yolk-sac of the young fish. 



In the stage last described (i. e., of plate vn, fig. 44), in spite of the advanced 

 characters of the embryo, the blastoderm has not increased vastly in size beyond 

 that shown in plate v, fig. 38. It has, however, as we see in plate vm, fig. 47, 

 constricted marginally, becoming cup-shaped, as it continues. to envelop the small 

 yolk mass. How far it has succeeded in inclosing the yolk 'is : p'erhaps better seen 

 in the details of the last figure, shown in figs. 47 a _and 47 b / ' /, : '''.''">'.',' ':'''. ''. A 



The relation of yolk and blastoderm is pictured in detail" in fig. 77, 'a section 

 passing through the blastoderm parallel to the long axis of the embryo. At the 

 points mb and mb', the rim of the blastoderm comes in contact with the yolk ; 

 above mb the blastoderm is thickened and 

 spongy; for, as a sign that the body of the 

 embryo lay adjacent, this region is richly 

 vascular. Noteworthy here is a deep sub- 

 marginal sinus (ms) whose posterior wall (c) 

 is cellular. We have in this condition a 

 physiological parallel with the submarginal 

 space in ganoids, and more directly even with 

 Kupffer's vesicle in teleosts. On the ventral 

 side of the blastodermic cap (on the left in 

 the figure) the vascular sponginess is largely 

 lost ; and the blastoderm is thin, save only at 

 its rim (mb'). And here in place of a deep 

 submarginal sinus, a number of distinct blood- 

 producing vesicles appear (ms) scattered 

 distally in a narrow zone of finely divided yolk (fy). From another standpoint, 

 finally, the present section is noteworthy. For it shows that the entire yolk-sac is 

 divided into masses which are largely separated from one another by a system 

 of fissure-like vacuoles. Closer inspection shows nuclei scattered irregularly 

 through these masses of yolk, and, everything considered, I think we can therefore 

 justly conclude that the yolk-sac at this stage, in spite of its relatively large size, is a 

 totally segmented structure comparable with the yolk-sac of Amia or Ichthyophis. 

 In the present case, it is true, the yolk masses (blastomeres) show a condition of 

 greater or less attachment to their neighbors, and each mass will usually contain 

 more than a single nucleus. But even in this event, the comparison will, I believe, 

 hold. In some cases the shape of the yolk masses is distinctly blastomere-like, 

 as between the vacuoles (v) in the present section. Viewed from this standpoint, 

 accordingly, Chimsera has retained a primitive embryological character, holoblastic 

 cleavage; but we can hardly fail to observe that this character has lost much of its 

 primitiveness inasmuch as the blastomeres are polynuclear and the intercellular 

 spaces obviously adapted as reservoirs of nutriment. 



Fig. 77. Section of extra-embryonic region and of upper 

 part of yolk-sac of stage of plate VIII, fig. 47. 



c. Cellular area ; mb, mb', margin of blastoderm: ms, marginal 

 sinus ; /.'/i fluid yolk ; V, vacuoles. 



